ColorpHast vs pH meter

Taste problems in my beers after I moved to a new house, had to use new water and started measuring mash pH with EMD's colorpHast strips prompted me to compare the pH determined by the pH strips with the pH measured by a pH meter. Though I later found that the taste problems had more to do with the hops I used I discovered an unexpected systematic error in the pH read from the pH strips which was also confirmed in an e-mail from EMD.

Contents

set-up

For the test 10 pH buffer solutions were mixed using the 4.01 and 7.01 calibration buffers that are sold by morebeer (MT610). The pH of these buffer solutions was chosen to match the pH values on colorpHast's color scale. The pH strips (MT629) and the pH meter (MT601) were purchased at morebeer.com

experiment

The pH meter was calibrated and each of the pH buffer samples were measured with the pH meter and the colorpHast strips. All measurements were taken at room temperature (70 *F / 20 *C). The colorpHast strips were submerged for 5 seconds until the color didn't change anymore. Leaving them in longer than that didn't change their reading. The strip's color was matched against the provided color scale in tungsten light. The color that matched the strip's color best was chosen as the reading and when a color seemed to fall between 2 colors the pH was read as the avarage of the pH given for the 2 colors on the color scale.

results

targeted buffer pH

actual buffer pH measured with pH meter

pH measured with colorpHast strips

difference

4.0 (4.01 buffer)

4.0

4.0

~ 0.0

4.4

4.38

4.0

~ -0.4

4.7

4.70

4.4

~ -0.3

5.0

5.01

4.7

~ -0.3

5.3

5.32

5.0

~ -0.3

5.5

5.50

5.15 *

~ -0.3

5.8

5.78

5.5

~ -0.3

6.1

6.14

5.8

~ -0.3

6.5

6.47

6.3*

~ -0.2

7.0 (7.01 buffer)

7.0

7.0

~ 0.0

) these samples read between 2 colors

The resuls show that the strips that were used in this experiment show a systematic error of about 0.3 pH units which meas that a mash which measures 5.3 with the strips is actually at pH 5.6. The subsequent lautering can be higher and can easily exceed the 5.8 maxium pH for tannin safe sparging

EMD's respose

I sent these results to EMD and wanted to get an input on these findings. Here is the e-mail that I got back:

Based on the information below, I am assuming that you are using the
4-7 clorpHast pH parer, part number 9582. The sensitivity of the part
number is 0.2-0.3 pH units. A pH meter will always be more accurate
than the pH strips ecause the meter will eliminate any human error in
determining color. Because of the possibility of human error, the strips
are consider semi-quantitative but are more convenient that a pH meter
when testing in the field and pH meter is not easily accessible. If you
need further assistance, feel free to contact me.